In recent years, the technology to manufacture a thin film transistor (hereinafter referred to as TFT) over a substrate makes great progress and application development to an active matrix display device has been advanced. In particular, TFT using a poly-crystal semiconductor film is superior in field-effect mobility (also referred to as mobility) to TFT using a conventional amorphous semiconductor film and thereby high-speed operation becomes possible. Therefore, it has been tried to control a pixel by a driver circuit formed over the same substrate as the pixel, which was controlled conventionally by a driver circuit provided outside the substrate.
By the way, a substrate used for the semiconductor device is expected to be a glass substrate rather than a single-crystal silicon substrate in terms of its cost. The glass substrate, however, is inferior in heat resistance and easy to change in shape due to the heat. Therefore, when a poly-silicon TFT is formed over the glass substrate, laser annealing is performed to crystallize the semiconductor film in order to prevent the glass substrate from changing in shape due to the heat.
The characteristic of the laser annealing is that the processing time can be drastically shortened compared with another annealing method by radiation heating or conductive heating and that a semiconductor substrate or a semiconductor film can be heated selectively and locally so that the substrate is hardly damaged thermally.
It is noted that the laser annealing method described herein includes the technique to recrystallize a damaged layer or an amorphous layer formed in the semiconductor substrate or the semiconductor film, and the technique to crystallize an amorphous semiconductor film formed over the substrate. In addition, the technique to planarize or modify the surface of the semiconductor substrate or the semiconductor film is also included.
The lasers used for the laser annealing are classified broadly into two types, a pulsed laser and a continuous wave (CW) laser according to its oscillation system. In recent years, it has been understood that in crystallization of the semiconductor film, a crystal grain formed over the semiconductor film becomes larger when using the CW laser than that when using the pulsed laser. When the crystal grain formed over the semiconductor film becomes larger, the number of the grain boundaries included in the channel region in TFT formed using this semiconductor film decreases and thereby the mobility becomes higher. As a result, such a semiconductor film can be applied to develop a device having higher-performance. For this reason, the CW laser oscillator is beginning to attract attention.
In addition, in order to enhance the productivity in the process of the laser annealing of the semiconductor or the semiconductor film by means of the CW laser, the method is often employed in which a laser beam emitted from the laser oscillator is shaped into long elliptical on the irradiated surface, and the elliptical laser beam (hereinafter referred to as the elliptical beam) is irradiated to the semiconductor film. The reason why the laser beam becomes elliptical is that the original shape of the laser beam is circular or near-circular. (For example, refer to patent document 1)
[patent document 1] Japanese Patent Unexamined Publication No. 2003-045820